Color plays an important role in all aspects of daily life, including a role in the identification of persons and things. One picks out the black haired lady in the red dress or the brown haired man wearing a blue suit from a crowd of people. Color serves also to communicate other information. The traffic signal provides a light to let one know whether to proceed, stop or prepare to stop with green, red and yellow illumination, respectively, the colors of which are obtained by inserting appropriate color filters in front of a multispectral light source, such as a white light. The red filter blocks the passage of all spectral components, except the red; the green filter blocks all spectral components, except green; and the yellow filter allows passage only of the yellow spectral component.
The illumination from colored lights provide a mood or ambience in theaters: The stage lights are adjusted to provide a light of a color that sets the mood or appearance desired by the playwright or to spot light a particular player on the stage. Color has application in the more esoteric fields, such as in identifying materials and gases by measuring the spectral absorption characteristics and comparing those to absorption characteristics of known materials and gases, a process referred to as "spectroscopy" and such as in electronic surveillance systems, in which the color of an object determines whether the object is one that is important or unimportant.
Electronic surveillance systems provide the means to monitor and display a field of view or scene that is placed under surveillance. The lay person is familiar with television cameras used by security forces to monitor and display secured areas and detect intrusion: Modern radar systems are used to provide general surveillance of the heavens for commercial purposes, such as in tracking commercial airplanes flying into and out of airports, and for military purposes, such as detecting and tracking war planes and, more importantly, the deadly self-guided missiles.
A common problem to all electronic surveillance systems is to separate the "clutter", those images representing static objects or even moving objects which, though in the field of view, are of no concern, from the target image or targets, which is the purpose of the surveillance. One may thus concentrate attention on the target. This known problem has been addressed in a combination of ways: by devising means to enhance the image of the desired target; to eliminate the "clutter"; and a combination of the two.
In critical military radar systems, achieving satisfactory clutter elimination is complicated many fold by the fact that the clutter itself in the field of view is moving and is numerous. For example, the reader may be familiar with newspaper articles that describe military threats posed by strategic attack missile systems, which includes the armed missile and decoys. The decoys are themselves missiles that are unarmed. In a hypothetical military threat, a large number of missiles are in effect "rained" upon a target location, only a few of which are armed. In a hypothetical countermeasure to this threat, a missile is to be used to shoot down the armed intruder. The object is to use the limited anti-missile resources available to shoot only at the live armed missile in an attempt to shoot the attacking missile down before it can do its deadly work. The numerous decoys serve as a device distracting the defenders from the real threat to their lives. Clutter rejection systems thus attempt to implement the 18th century command "Don't Shoot Before You Can See the Whites of Their Eyes", in a 20th century context. Although means exist to allow military defenders to detect which missile is armed and which is the decoy, the details of those "footprints" or "signatures" are not relevant to the description of the present invention and need not be addressed, except in the context they share with other kinds of clutter, both dynamic and static; that characteristic is spectral, the color associated with the object.
By way of example, one may consider an image in which a decoy missle is colored red, while the true armed missle appearing in the same field of view, is represented in the color blue. By filtering out the red as unimportant or "clutter", the position and movement of that armed missle may be viewed without distraction and the defenders guns may be properly aimed.
Digital computers have been used to acquire track surveillance information and to enhance the display by electronically eliminating the display of unwanted objects. However, as the amount of such clutter and its movement in the field of view increases, the processing capabilities of computers must be increased, requiring more expensive and faster computers with greater memory. Situations are foreseeable in which existing computers may be insufficient as a practical matter to handle a scene containing hundreds of unwanted moving objects. An advantage of the invention is that a great deal of the clean up work in this environment is handled optically. This reduces the amount of information that the main computer is called upon to handle.
A form of spectral preprocessing is known. In this various color filters are mounted on a wheel that is rotated at high speed so as to take rapid consequtive snap shots of the field of view as perceived in separate colors. The wheels rotation is synchronized with associated electronic cameras, such as a video camera so that the individual snap shots are taken, digitized and stored in an associated computer where the information is acted upon. This mechanical flying spot scanner type of device is obviously limited to a small number of possible filters and in a sense offers little benefit in reducing computer needs in fields under surveillance containing large amounts of moving clutter. An advantage of the present invention is that the color filtering combinations appear to be virtually unlimited.
An object of the invention is to provide an optical preprocessor to modify the spectral content of a received image. A further object is to provide a preprocessor system to optically remove clutter from received images in electronic surveillance systems. A still further object of the invention is the provision in a spectral modifying device of a novel mirror having controllable reflectivity and to provide multispectral light systems having means using controlled reflectivity mirrors for providing measurement, detection, and communications.